A natural lighting system (including a solar light condensing system) which has been developed up to date or has been commercially used can be classified into a fixed-type lighting system using an optical duct and a light condensing system using a solar light tracking lens (a spherical lens or a fresnel lens). In the fixed-type lighting system using an optical duct, efficiency of condensing solar light is lower than that of in solar light tracking, but lighting is possible regardless of a sky condition (a weather state). On the other hand, in the light condensing system using solar light tracking, lighting is possible in a clear sky condition or an overcast sky condition and light condensing efficiency is high.
Excellence of both lighting systems can be hardly compared. With regard to this, the fixed-type lighting system is used in indoor lighting and the light condensing system using solar light tracking is used in local area lighting in indoor.
In particular, the natural lighting system using solar light tracking is divided into a reflective mirror type (a plane or a curved reflector) and a lens type, based on a principle of light condensing, and is divided into a reflective mirror type and an optical fiber type, based on a light transmission method.
In the reflective mirror type, solar light is transmitted by a reflective mirror without separately using a light condenser and thus, long distance transmission for solar light is possible. However, a size of the reflective mirror and a space for light transmission should be sufficiently secured. In the lens type, there are limitations in a light transmission distance (within 30 m) and in economical efficiency of the optical fiber due to light transmission using the optical fiber, thereby having no practical use.
In addition, the conventional solar light condensing system is installed to a roof of a building. However, in environment where high buildings with more than five stories (including apartments) as in Korea, there is a great need of installing the solar light system on outer walls of the buildings.
The conventional solar light condensing system has a lens or a reflective mirror used in condensing solar light which always face solar light vertically. However, due to the climate and environmental characteristics in Korea located at an solar altitude of between 30 degrees and 70 degrees, all solar light condensing system using solar light tracking must be separately installed on surfaces of roofs of the buildings. When the solar light condensing system is installed to outer walls of the buildings, light condensing is difficult and efficiency of light condensing is nearly 0 and thus, the solar light condensing system is hardly used.
In addition, when the solar light condensing system is installed to roofs of the high buildings, there is a limitation in a distance for transferring condensed solar light and difficulties in making good appearance of the buildings and managing the system.